Electrostatic printing process and apparatus



Jan. 14, 1964 H. G. GREIG 3,117,884

ELECTROSTATIC PRINTING PROCESS AND APPARATUS Filed March 25, 1955Iffar/w/ United States Patent 3,117,884 ELECTROSTATIC PRINTING PROCESSAND APPARATUS Harold G. Greig, Princeton, N.J., assignor to RadioCorporation of America, a corporation of Delaware Filed Mar. 23, 1955,Ser. No. 496,262 14 Claims. (Cl. 117-175) This invention relates toelectrostatic printing and particularly, but not necessarilyexclusively, to improved methods and means for developing electrostaticcharge patterns and to improved electrostatic printing processes andapparatus.

An electrostatic printing process is that type of process for producinga visible record, reproduction or copy which includes as an intermediatestep, converting a light image or electrical signal to a latentelectrostatic charge pattern on an electrically-insulating substrate.The process may also include converting the charge pattern into avisible image which may be a substantially faithful reproduction of anoriginal except that it may be different in size and color.

A typical electrostatic printing process may include first producing auniform electrostatic charge upon the surface of a photoconductiveinsulating material, such as selenium, anthracene, or zinc oxidedispersed in an electrically-insulating, film-forming vehicle. Then, thecharged surface of the photoconductive material is exposed to a lightimage incident upon the charged surface, discharging the portionsirradiated by the light rays, while leaving the remainder of the surfacein a charged condition, thereby forming a latent electrostatic imagesubstantially corresponding to the light image. The latent electrostaticimage is developed by applying a developer powder which is heldelectrostaticaliy to selected areas of the surface. The visible powderimage thus formed may be fixed directly to the photoconductive surfaceor it may be transferred to another surface upon which the visible imagemay be desired and then fixed thereon.

A typical developing technique utilizes a magnetic brush comprising adeveloper mix including magneticallyresponsive carrier particles anddeveloper powder particles maintained in a loose mass by a magneticfield. When the loose mass is swept across the surface bearing a latentelectrostatic image, the magnetic particles carry the developer powderparticles to the desired point of contact and at the same time, providean electrostatic charge upon the developer powder particles bytriboelectricity. The electrostatically-charged developer powderparticles brought into contact with the surface bearing the latentelectrostatic image are retained on selected areas of the surface byelectrostatic attraction in substantial configuration with theelectrostatic image.

An object of the invention is to provide improved methods and means ofelectrostatic printing.

Another object is to provide improved methods and means for developinglatent electrostatic charge patterns and electrostatic images with amagnetic brush.

Another object is to provide an improved magnetic brush forelectrostatic printing.

Another object is to provide improved methods and means for developingeither a direct or reverse image from a single latent electrostaticimage.

A further object is to provide improved methods and means forcontrolling the contrast value of the visible powder image and tocontrol spurious deposit of developer powder particles in the backgroundof a visible powder image developed with a magnetic brush.

In general, the processes of the invention comprise establishing aunidirectional electric field between a latent 3,117,884 Patented Jan.14, 1964 electrostatic image and a magnetic brush during the developmentof said latent electrostatic image. By varying the strength anddirection of the electric field, the contrast of the developed image maybe varied, the developed image may be reversed and the spurious depositof developer powder particles in the background of the developed imagemay be controlled. Apparatus in accordance with the invention includes amagnetic brush and means for establishing a desired unidirectionalelectric field between said brush and an electrostatic image during thedevelopment of said electrostatic image.

The foregoing objects and other advantages will be more fully describedin the following detailed description when read in conjunction with theaccompanying drawing in which:

FIGURE 1 is a partially sectional, partially schematic view of a firstimproved apparatus for carrying out the processes of the invention,

FIGURE 2 is a partially sectional, partially schematic view of a secondimproved apparatus for carrying out the processes of the invention, and

FIGURE 3 is a sectional view along section lines 3-3 of FiGURE 2.

Similar reference characters are applied to similar elements throughoutthe drawing.

Referring to FIGURE 1, a first improved apparatus for carrying out theprocess of the invention comprises a. developer brush adapted to becontacted across the surface of an electrically-insulating layer bearinga latent electrostatic image. The developer brush comprises a magneticpole piece 21, a mass of developer mix 23 includingmagnetically-responsive carrier particles and developer powder particlesheld together and to the pole piece 21 by magnetic attraction andconnection means 25 for a source of biasing voltage attached to the polepiece 21.

A preferred carrier material for the developer mix consists ofalcoholized iron, that is, iron particles free from grease and otherimpurities soluble in alcohol. These iron particles are preferablyrelatively small in size, being in their largest dimension about 0.002to 0.008 inch. Satisfactory results are also obtained using a carrierconsisting of iron particles of a somewhat wider range of sizes up toabout 0.001 to 0.020 inch.

A preferred developer powder may be prepared as follows: a mixturecomprising 200 grams of 200 mesh Piccolastic resin 4358 (an elasticthermoplastic resin composed of polymers of styrene, substituted styreneand its homologs), marketed by the Pennsylvania Industrial Company,Clairton, Pa, and 12 grams of Carbon Black G marketed by the Elmer andAmend Co., New York, N.Y. are thoroughly mixed in a stainless steelbeaker at about 200 C. The mixing and heating should be done in as shorta time as possible. The melt is poured upon a brass tray and allowed tocool and harden. The hardened mix is then broken up and ball milled forabout 20 hours. The powder is screened through a 200 mesh screen and isthen ready for use as a developer powder. This powder takes on apositive electrostatic charge when mixed with iron powder. It thereforewill develop the negatively charged areas of an electrostatic image.About 2 to 10 grams, but preferably 5 grams, of the developer powder andabout grams of the magnetic carrier material are blended together givingthe preferred developer mix. Other ratios of developer powder tomagnetic carrier material may be used.

The magnetic pole piece 21 comprises an iron bar with a permanentlymagnetized pole at one end thereof. The magnetic pole produces anexternal magnetic field which attracts and holds the mass of developermix 23. The attracted mass of developer mix is loosely held and easilydeformable to the contours of a surface with which it s,117,ss4

3 may be in contact. At the other end of the iron bar, there is provideda threaded hole and screw for connecting a source of biasing voltage tothe developer brush.

A battery 32l or other source of biasing voltage is connected to themagnet 21 through the connection means 25, a double pole, double-throwreversing switch 33 and a potentiometer 35. Thus, the applied biasingvoltage may be changed in polarity and varied in magnitude.

An electrically-insulating layer 41 bearing a latent electrostatic imageis placed upon a grounded conducting backing plate 43. The switch 33 andthe potentiometer 35 are adjusted such that a positive voltage of about700 volts with respect to ground is applied to the magnet 21, and thedeveloper mix 23 of the brush is contacted across the surface of thelayer 41. During the period of contact, a unidirectional field appearsbetween the mass of developermix 23 and the backing plate-43 due to thebiasing voltage applied between the brush and the backing plate 43. Theunidirectional field is in addition to the electric field of the latentelectrostatic image. Developer powder particles 67 from the mass ofdeveloper mix 23 deposit in areas of the layer 41 having a negativeelectrostatic charge producing a visible powder image thereon.

The visible powder image produced by the abovedescribed procedure has avery high contrast characteristic and a minimum amount of spuriousdeposit in the background areas. This set of characteristics isconsidered ideal for line drawings and line prints. By reducing thebiasing voltage and therefor the unidirectional field, the contrastbetween the dark and light areas of the image is reduced, making itpossible to obtain any desired contrast characteristic over a very widerange of contrast values. When the voltage is reduced to zero, themagnetic brush as heretofore described will develop the negativelycharged areas of the layer 41. However, a moderate amount of developerpowder frequently deposits spuriously in the background areas of thevisible image. Such spurious deposit may be reduced to a minimum valueby biasing the magnetic brush according to the method of the invention.

If the switch 33 is reversed and an increasing voltage applied to themagnet 21, a value is reached where the negatively charged areas of thelayer 41 are no longer developed, and instead the positively chargedareas of the layer 41 are developed producing areverse image. Thecontrast of the reverse image increases and the amount of spuriousdeposit in the background areas of the reverse image decreases as thenegative voltage is increased in magnitude.

According to the invention, a direct or reverse visible image may beobtained from the same latent electrostatic image. A direct visibleimage is a developed image wherein the developed areas correspond to thedark areas of the original. Where a photoconductive process is utilizedto produce the latent electrostatic image, the developed areas of thevisible image are the unexposed areas of the photoconductive layer. Areverse visible image is a developed image wherein the developed areascorrespond to the light areas of the original. Also, by adjustment ofthe applied voltage below the dielectric breakdown of the layer in whichthe latent electrostatic image resides, any desired contrastcharacteristic over a wide range may be obtained for either the positiveor the reverse image. Thus, one may simply and quickly adjust anelectrostatic printing apparatus to produce line prints of high contrastvalue, continuous tone prints of intermediate contrast value and, ineach case, the print may be direct or reverse.

The developer powder particles of the developer mix may be chosen from alarge class of materials, for example; zinc, copper, carbon, sulphur,gum copal, gum sandarac, nylon, polystyrene, sealing wax and othernatural or synthetic resins or mixtures thereof. The developer powderparticles may be coated with a thin layer of a material for the purposeof modifying the physical or electrical properties of the developerpowder. It is preferred, however, to use a pigmented thermoplasticsynthetic resin.

The magnetic carrier particles of the developer mix may be chosen from alarge class of powdered magneticallyattractable materials such as iron,steel, alloys of aluminum, nickel and cobalt and other magneticmaterials.

The developer powder and the magnetic carrier powder may be combined inany desired proportion to produce the developer mix provided themagnetic carrier particles are able to carry the developer powderparticles. It is preferred that 2 to 10 parts of developer powder bemixed with parts of magnetic carrier powder.

A quantity of the developer mix maintained in a loose mass by a magneticfield and means for establishing a unidirectional electric field betweenthe developer mix and the latent electrostatic image during thedevelopment of said image are the essential structural components of theimproved magnetic brush of the invention. It is preferred to utilize apermanent bar magnet for providing the magnetic field for maintainingthe developer mix in a loose mass and voltage connection means attachedto the bar magnet'for electrical connection to the developer mix.However, other structures may be used, such as electromagnets or othermagnetic field producing means. Similarly, the biasing voltage may beapplied to the mass of developer mix of the brush as shown in FIGURE 1or may be applied to the backing plate 43 with the developer connectedto ground, as subsequently described in FIGURE 2.

Any positive or negative oltage may be used to bias the magnetic brushof the invention so long as there is no electrical breakdown in thebrush or in the layer 41 upon which the electrostatic image resides. Itis preferred, however, to use a voltage between -l000 and +1000 voltsDC.

The theory underlying the processes of the invention is not completelyunderstood. It is believed that a virtual ground plane is automaticallyand inherently provided by the magnetic brush immediately adjacent tothe electrostatic image. The extreme closeness of the virtual groundplane to the latent electrostatic image greatly enhances the image fieldeffect. By applying an electric field in the desired polarity inaddition to the image field, the image field is effectively furtherenhanced or suppressed, to produce a wide range of direct or reversecontrast values in the developed visible image. This effect may be dueto contact charging of the developer powder particles at the brush endsor to an electric field additional to the electrostatic image field orboth.

Referring to FIGURES 2 and 3, the improved methods and means of theinvention may be embodied in an improved continuous electrostaticprinting process and apparatus. A continuous web comprising a papersubstrate 51 having on one surface thereof a photoconductive insulatingcoating 53 comprising a powdered photoconductor, such as zinc oxide,dispersed in an electrically-insulating, film-forming vehicle is unwoundfrom a roll 55. The continuous web first passes a station where auniform negative electrostatic charge is produced on the photoconductivecoating 53, for example, by corona discharge from a series of wires 57connected to a voltage source 59 through a double-pole, double-throwreversing switch 61 and a potentiometer 63. The surface of the coating53 may also be charged by rubbing or by contact with a biased conductiveroller.

The continuous web next passes to a station where an electromagneticradiation image is produced upon the surface of the photoconductivecoating 53, for example, by projection from a photographic transparencyimage by means of a projector 65. The continuous web now has producedthereon a latent electrostatic image substantially corresponding to theelectromagnetic radiation image which was projected thereon.

The continuous web next advances to a station where the latentelectrostatic image residing on the surface of the photoconductivecoating 53 is developed by the method of the invention. A groundedrotary pole piece of a magnetic structure is provided with spacedparallel inclined elliptical discs 22 facing the photoconductive coating53. A magnetic field is maintained between a fixed magnetic pole piece45 spaced from the elliptical discs 22 and on the opposite side of theweb through magnetic pieces 47 and 49 and through the gap therebetween.A reservoir 71 holds a quantity of developer mix in contact with thediscs 22. A shield 44 located between the fixed pole piece and the webis maintained in contact with or closely spaced behind the papersubstrate 51 and is connected to a voltage source 31 through adouble-pole, double-throw reversing switch 33 and a potentiometer 35.The switch 33 and the potentiometer 35 are adjusted to provide thedesired biasing voltage upon the shield 44 for example, a positivevoltage of 700 volts. As the rotary pole piece 20 rotates in a clockwisedirection, developer mix forms on the periphery of the discs 22 inbrush-like filaments 23 and is carried upwardly and swept across thesurface of the photoconductive coating 53 passing the station. Developerpowder particles 67 deposit upon areas of positive polarity producing areverse visible powder image on the photoconductive coating 53 insubstantial configuration with the latent electrostatic image. Accordingto the invention, direct or reverse visible powder images may beproduced and the background color value and the contrast value of thevisible image may be varied by adjusting the biasing voltage to theshield 44 as previously described in FIGURE 1.

The continuous web bearing the visible powder image thereon now passesto a station where the visible image is fixed to the photoconductivecoating 53. For this purpose a radiant heater comprising a resistancewire 73 connected to a voltage source 75 through a potentiometer 77 ismaintained in closely spaced relationship with the visible powder image.Heat radiated from the wire 73 softens the thermoplastic resin of thedeveloper powder causing it to adhere to the photoconductive coating 53.The visible powder image may be fixed to the photoconductive layer 53 byother means for example, by spraying with an adhesive or by coating witha softener for either the photoconductor 53 or the developer powderparticles. The visible powder image may also be transferred to anothersurface and fixed thereon by any convenient means. The continuous webbearing the fixed visible image is now wound upon the roll 56. Thecontinuous web may of course be cut into convenient lengths and stackedin piles or utilized directly.

There have been described improved methods and means of electrostaticprinting including improved meth' ods and means for developing latentelectrostatic charge patterns and electrostatic images with an improvedmagnetic brush. There have also been described methods and means forproducing direct or reverse visible images and for controlling thecontrast value of the visible powder image developed according to animproved electrostatic printing process.

What is claimed:

1. A process for developing a latent electrostatic image residing upon asurface comprising contacting said latent image with a mix comprisingmagnetic carrier particles and developer powder particles, said mixbeing maintained in a mass by a magnetic field and establishing betweensaid mass and said surface a unidirectional electric field in additionto the electric field emanating from said electrostatic image.

2. A process for developing a latent electrostatic image residing uponthe surface of an electrically-insulating layer comprising contactingsaid surface with a mix comprising magnetic carrier particles anddeveloper powder particles, said mix being maintained in a mass by amagnetic field and establishing through said layer to said mass a uni- 6directional electric field in addition to the electric field emanatingfrom said electrostatic image.

3. In electrostatic printing, a method for developing a latentelectrostatic image residing in an insulating layer comprisingcontacting with one surface of said layer a developer brush comprising aquantity of magnetic particles intermixed with a quantity of coloreddielectric particles said magnetic and dielectric particles beingmaintained in a loose mass by a magnetic field, spacing an electrodefrom said brush and on the side of said layer opposite to said brush andapplying a voltage between said electrode and said developer brush.

4. In electrostatic printing, a method for developing a latentelectrostatic image upon a first surface of an insulating sheetcomprising securing a mass of magnetic developer mix in a magnetic fieldto form a developer brush, contacting said developer brush across saidfirst surface, contacting an electrode to said sheet on a second surfaceopposite to said first surface and applying a voltage between saidelectrode and said mass of developer mix.

5. In an electrostatic printing process comprising the steps of formingan electrostatic image and then applying to said electrostatic image amass of a developer mix including magnetically-responsive carrierparticles and developer powder particles held together and to a magneticpole piece by magnetic attraction, the step comprising applying anelectric field between said electrostatic image and said mass ofdeveloper mix to control the attraction between said developer powderparticles and said electrostatic image.

6. Apparatus for developing a latent electrostatic image comprisingmeans for contacting an electrically-conducting mass of powder particlesincluding developer powder particles across a surface bearing saidelectrostatic image, and an electric circuit for applying to said mass avoltage of a predetermined magnitude and polarity with respect to theregion immediately behind said surface.

7. A method for developing a latent electrostatic image comprisingcontacting an electrically-conducting mass of powder particles includingdeveloper powder particles to the surface bearing said latentelectrostatic image, and simultaneously applying to said mass anexternal voltage of a predetermined magnitude and polarity with respectto the region immediately behind said surface.

8. Apparatus for developing an electrostatic image on a surface throughthe use of a magnetically responsive developer mix comprising magneticfield producing means for forming said developer mix into a magneticbrush and electrical connection means for applying a biasing voltagebetween said magnetic brush and said surface.

9. A device for developing electrostatic images residing upon a surfaceof a sheet through the use of a developer mix including carrierparticles of magnetic material intermixed with developer powderparticles, said device comprising magnetic field producing means formaintaining said developer mix in a loose mass and means forestablishing an electric field in addition to the electric fieldemanating from said electrostatic image between said mass and saidsurface when said mass is contacted to said surface.

10. A device for developing electrostatic images through the use of adeveloper mix including carrier particles of magnetic materialintermixed with a colored developer powder of a different material, saiddevice comprising a magnet having an external magnetic field for formingsaid developer mix into a brush in contact with a pole of said magnetand means connected to said magnet for producing in said brush a desiredbiasing voltage with respect to some fixed voltage.

11. A device for developing electrostatic images residing in a sheet ofinsulating material through the use of a developer mix includingmagnetic carrier particles intermixed with developer powder particles,said device comprising means for producing a magnetic field to form saidmix into a brush, means for contacting said brush across one surface ofsaid sheet, an electrode in contact with the opposite surface of saidsheet, and means for applying a voltage between said electrode and saiddeveloper mix.

12. A device for developing a latent electrostatic image on a surfacethrough the use of a developer mix including magnetically-responsivecarrier particles and developer powder, said device comprising amagnetic pole piece for forming said mix into a loose mass and forholding said mix in contact with said pole piece by magnetic attractionand electrical connection means for applying to said pole piece a biasvoltage with respect to said surface.

13. In an electrostatic printing device, apparatus for developing latentelectrostatic charge patterns residing in an insulating layer throughthe use of a developer mix including magnetic carrier particles anddeveloper powder particles, said apparatus comprising magnetic means forforming said mix into a loose mass and for contacting said mix acrossone surface of said layer and means for establishing a unidirectionalelectric field transversely through said contacted surface.

14. In apparatus for developing an electrostatic image on a surfaceincluding a magnetic field-producing, electrically-conductive member forforming a magneticallyresponsive developer mix into a magnetic brush,the improvement comprising an electrical connection associated with saidmember for applying to said magnetic brush a bias voltage with respectto said surface.

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1. A PROCESS FOR DEVELOPING A LATENT ELECTROSTATIC IMAGE RESIDING UPON ASURFACE COMPRISING CONTACTING SAID LATENT IMAGE WITH A MIX COMPRISINGMAGNETIC CARRIER PARTICLES AND DEVELOPER POWDER PARTICLES, SAID MIXBEING MAINTAINED IN A MASS BY A MAGNETIC FIELD AND ESTABLISHING BETWEENSAID MASS AND SAID SURFACE A UNIDIRECTIONAL ELECTRIC FIELD IN ADDITIONTO THE ELECTRIC FIELD EMANATING FROM SAID ELECTROSTATIC IMAGE.
 11. ADEVICE FOR DEVELOPING ELECTROSTATIC IMAGES RESIDING IN A SHEET OFINSULATING MATERIAL THROUGH THE USE OF A DEVELOPER MIX INCLUDINGMAGNETIC CARRIER PARTICLES INTERMIXED WITH DEVELOPER POWDER PARTICLES,SAID DEVICE COMPRISING MEANS FOR PRODUCING A MAGNETIC FIELD TO FORM SAIDMIX INTO A BRUSH, MEANS FOR CONTACTING SAID BRUSH ACROSS ONE SURFACE OFSAID SHEET, AN ELECTRODE IN CONTACT WITH THE OPPOSITE SURFACE OF SAIDSHEET, AND MEANS FOR APPLYING A VOLTAGE BETWEEN SAID ELECTRODE AND SAIDDEVELOPER MIX.